1. Field of the Invention
The present invention relates to a developing device utilizing an electrophotographic method and an image forming apparatus.
2. Description of the Related Art
Generally, an image forming apparatus that utilizes an electrostatic photography method forms an image by charging, exposing, developing, transferring, cleaning, charge neutralizing, and fusing processes. For example, in the processes of forming an image, an electrostatic latent image is formed as a result of, uniformly charging a surface of a rotating drum type of photoconductor by a charging device, and irradiating the surface of the charged photoconductor with laser light by an exposure device.
Next, the electrostatic latent image on the photoconductor is developed by a developing device to form a toner image on the surface of the photoconductor. The toner image on the photoconductor is transferred onto a transfer material by a transfer device, and then, the toner image is fixed onto the transfer material as a result of pressure and heat being applied by a fusing device. The residual toner remaining on the surface of the photoconductor is removed by a cleaning device and collected in a collection section of the cleaning device. In addition, the residual charge is removed from the cleaned surface of the photoconductor by a neutralization device in order to prepare the next image formation.
Generally, a mono-component developer including only a toner, or a two-component developer including a toner and a carrier is used as a developer for developing the electrostatic latent image on the photoconductor. Since the carrier is not used in the mono-component developer, it is not necessary to have an agitating mechanism or the like in order to uniformly mix the toner and the carrier. Thus, the mono-component developer has the advantage of allowing the design of the developing device to be simplified. On the other hand, the mono-component developer has the disadvantage that it is difficult to stabilize the toner in electric change amount.
Since the two-component developer needs an agitating mechanism or the like for uniformly mixing the toner and the carrier, the two-component developer has the disadvantage of requiring a complicated design for the developing device. However, as a result of being superior in stabilizing the electric charge amount, the two-component developer is often used in a high-speed image forming apparatus or a color image forming apparatus.
In order to meet the demands of color printing, high-speed printing, and energy saving, there have been progressed in reduction of a particle size and a softening temperature of the toner used in the two-component developer. However, such a toner has the disadvantage of having a tendency to aggregate due to heat. Thus, if the temperature within the developing device rises due to frictional heat caused during agitation in the developing device, the temperature of the developer is increased. This leads to problems that the image is unevenly formed due to the aggregation of the developer and the reduction in fluidity of the developer.
Known methods that solve this problem include a method of cooling the developer by supplying air to the developing device, and or method of cooling the developer by installing a cooling element to the developing device (e.g., see Japanese Unexamined Patent Application No. HEI 01-219854).
However, the method of cooling the developer by supplying air has a problem where cooling capacity decreases when the surrounding temperature of the image forming apparatus is high. Furthermore, the method of cooling the developer by the cooling element has the following problems. The condensation occurs inside the developing device due to excessive cooling resulting from a high cooling capacity; and it is difficult to keep the temperature of the developing device constant even when the temperature control is conducted by a temperature detection sensor.